Create a public interface for getting server info

[danielbachhuber]

Rather than having the following throughout:

$server = array(
    'ping_status' => $ping ? 'successful' : 'failed',
    'ip_address'  => getenv( 'PANTHEON_INDEX_HOST' ),
    'port'        => getenv( 'PANTHEON_INDEX_PORT' ),
    'path'        => SolrPower_Api::get_instance()->compute_path(),
);

We should have a method on the API class that returns this data, for use by the options page and CLI.